Scope of workbook in write excel

[devbond]

I want to declare workbook outside the subroutine because i want to use it in other subs also .but the following code is not working for me. Please Help.

#!/usr/bin/perl
use warnings;
use Spreadsheet::WriteExcel;
use Spreadsheet::ParseExcel;
use Spreadsheet::ParseExcel::SaveParser;
my $workbook;
my @s;
$Month=Sept;


sub create_excel {

$workbook = Spreadsheet::WriteExcel->new('FAISTATS_'."$Month".'.xls');
for ( $i=1;$i<=8;$i++)
{
$s[$i] = $workbook->add_worksheet( "cpzea0".$i."a0001" ) ;
}
$s[9] = $workbook->add_worksheet( "WEEKLY" ) ;
$s[10] = $workbook->add_worksheet( "MONTHLY" ) ;
}
create_excel();